Hi Jazz Cat
I get a warbly sound playing the low C with my Guardala MB II fat boy tenor. Tip opening is 8* or 116. When I blow my 7* mouthpieces there is no warble. I assume the larger tip opening contributes to a warble wound.
sounds weird to me….did you try going from both octaves? I would experiment that way, and also going from thos notes, and going to those notes to see if it is indeed just those notes cause it doesn’t make sense